A variety of quality control mechanisms operate in the secretory pathway to ensure the fidelity and regulation of protein expression during cell life and differentiation. As a rule, only proteins that pass a stringent quality control process are transported to their target organelles and compartments. If proper maturation fails, the aberrant products are degraded. Quality control improves folding efficiency by retaining proteins in the environment of the endoplasmic reticulum until proper folding, and prevents harmful effects caused by the release of incompletely folded or assembled proteins. The secretory pathway, therefore, can be used to assist in proper folding and assembly of a heterologous polypeptide.